A conventional occupant protection apparatus prevents operation error even if a vehicle is submerged. The device is disclosed in JP-A-2006-131230. In JP-A-2006-131230, the device is an air bag system for a vehicle including a submersion detection unit. When the water penetrates in the vehicle after the vehicle is submerged, the submersion detection unit provides to cut power supply to an output control unit. Therefore, even if an inflation signal to the air bag is accidentally output in case of water penetration, the system prevents air bag inflation.
However, in the above system, it is necessary to add the submersion detection unit in the air bag system. Thus, the number of parts of the system increases, and a manufacturing cost of the system increases.